Quantum dots provide a functional platform for novel materials and devices that utilize the unique physical properties that arise from their quantum confined nature. For semiconductor quantum dots such as cadmium selenide, CdSe, variation of particle size leads to continuous and predictable changes in fluorescence emission. Such quantum dots are under investigation as the basis for new materials and devices including photovoltaic cells, light emitting diodes, bio-sensors, and hybrid materials prepared by directed-and self-assembly techniques. However, exploitation of quantum dots in such applications requires an appropriate manipulation of their solubility or miscibility within the host environment. One approach involves tailoring of a ligand periphery on the quantum dot surface.
Seminal efforts and subsequent studies centered on the preparation of quantum dots have provided high quality samples covered with tri-n-octylphosphine oxide (TOPO) ligands to give a hydrophobic and chemically inert ligand shell. Use of functionalized ligands in the nanoparticle synthesis is generally precluded due to instability of such ligands at temperatures needed for the growth of high quality quantum dots. For example, conversion of TOPO-covered quantum dots to water-dispersible materials invariably requires alternatives to the TOPO periphery, typically accomplished by a ligand exchange. Recent efforts towards functionalization of quantum dots include the use of polymers, oligopeptides, oligonucleotides, and electronically active materials. However, there remain significant challenges associated with the use of ligand exchange, as surface oxidation, changes in quantum dot size and size-distribution, and diminished photoluminescence often accompany these chemistries. Nevertheless, ligand-exchange remains standard practice in the art for the introduction of new surface functionality to quantum dots.
The integration of CdSe quantum dots into electronically active polymer matrices is leading to a new generation of devices such as photovoltaic cells and light emitting diodes. Numerous advances, as well as difficulties, related to the fabrication of such devices from nanoparticle-based composites have been encountered. Previous work in this area utilized simple physical mixing of conventional quantum dot materials with electronically active polymers such as polythiophene and poly(para-phenylene vinylene). One key problem is centered at the polymer-quantum dot interface. Typically, as discussed above, the quantum dots are covered with, or bound by, TOPO, pyridine or other such surface binding or chelating ligand(s). The insulating TOPO-coverage limits charge transport between the quantum dots and the surrounding polymer matrix. Furthermore, the use of either TOPO-covered or TOPO-free (“stripped” or pyridine covered) quantum dots leads to nanoparticle aggregation within the matrix. Diminished interfacial interactions lead to an apparent self-quenching of nanoparticle emission, compensation for which comes with a cost of very high nanoparticle loading (e.g., 50-90 wt. percent).